1. Technical Field of the Invention
The present invention relates in general to lithographic exposure processes such as electron beam lithography. It relates more specifically to projection lithography systems employing a scattering mask instead of the conventional blocking masks and to exposure nonuniformities caused by energy scattering within or near the target of such lithographic processes. The invention is particularly applicable to the field of electron beam lithography for integrated circuits but is also beneficial for projection systems using other types of energy, such as X-rays or ultraviolet light, or having other purposes, such as production of integrated optics or microscopic mechanical devices.
2. Terminology
To avoid ambiguity or confusion, it is appropriate to define some of the terms used in the present application. The following definitions are made as clear as possible by limiting them to simple electron beam lithography of integrated circuits, but analogous definitions would apply to X-rays or other forms of radiation, to applications other than the patterning of integrated silicon circuits and to other generalizations. The specificity of these definitions is not intended to limit the generality of the invention.
Transparent: Passes electrons with little or no scattering.
Projection Mask: A master pattern of transparent and nontransparent areas which determines where electrons will and will not strike the target. Usually the master pattern is projected onto the target at a significantly reduced size.
Scattering Mask: A projection mask in which the nontransparent areas generally scatter the electrons somewhat (add relatively small random deviations to their original vectors) instead of absorbing or reflecting them.
Scattering-mask Lithographic Projection System: An electron beam system incorporating, in sequence: an electron source, a scattering mask, a back focal plane where electrons spreading directly from the source would be focused, and a target plane where electrons spreading directly from the scattering mask would be focused. Such a system also requires, as a minimum, one lens (generally in the vicinity of the scattering mask) to focus the source at the back focal plane, and one lens (generally in the vicinity of the back focal plane) to focus the mask at the target.
Back Focal Plane Filter: A mask of transparent and nontransparent areas, placed at a plane where the original electron source is imaged. Such a filter can be designed to pass, scatter or block electrons striking it in different places.
Target: A silicon wafer coated with a "resist" material that becomes more easily dissolved when exposed to an electron beam. Thus, after chemical processing the resist material is entirely removed from all areas of the target where the projected electron beam exposure was sufficiently intense. Because of uncontrollable variations in the chemical processing, faithful reproduction of the intended pattern is possible only when there is a significant difference between the minimum exposure in areas intended to be exposed, and the maximum exposure in areas intended to remain unexposed.
Backscattering: The scattering of incident electrons within the silicon of the target, so that they re-emerge and produce additional resist exposure a significant distance from the original impact point. The accelerating voltage of the electrons has a significant effect on the backscatter distribution profile, which may extend only a few micrometers or tens of micrometers from the impact point.
Proximity Effect: A pattern-dependent variation in general exposure levels. Electrons passing through the resist and striking the underlying silicon substrate are scattered back upward, producing additional resist exposure on their second pass. Because the backscattered electrons typically spread over an area much larger than a minimum-size pattern feature, an isolated spot may get only 5% or less additional exposure; however, accumulated backscattering from many adjacent spots may increase effective resist exposure by 80% or more.
Using those nominal percentages and assuming that direct exposure is 100 units, it can be seen that large exposed areas would receive 1130 units, small unexposed areas in the middle of such large exposed areas would receive 80 units, and small isolated exposed areas would receive 105 units of exposure. That 105-to-80 difference is not a comfortable margin for process control. The proximity effect causes dense pattern areas to be overexposed and/or sparse pattern areas to be underexposed.